1. Field of the Invention
This invention relates to mechanisms which transport thermal energy and more particularly, is concerned with a type of closed system in which the thermal transfer is accomplished primarily by movement of working fluid mass in a liquid-vapor phase-change cycle, and the motive power causing the device operation is thermal in nature.
2. Discussion of Prior Art
The increasing importance of thermal transport technology is indicated by the broadening of the areas of its application. Manufacturing processes, heat recovery, electronics cooling, air conditioning, solar energy collection, motor and engine cooling, and thermal management in outer-space structures, share a common requirement; to wit, that systems be installed which will facilitate and control the transportation of thermal energy.
Consider two regions which exhibit a measurable difference in temperature, a hot zone termed heat-source and a cold zone termed heat-sink. There are requirements to move thermal energy over separating distances and around obstructions, from the source to the sink, using a thermal-powered devise. Certain phase-change thermal transport devises have been developed in order to satisfy these requirements.
In a phase-change system the thermal-powered vapor phase circulation, i.e. the evaporation of a liquid, transport of the vapor through a duct, and subsequent condensation, is a well known method for transporting thermal energy. This action is efficient and consistently employed in all such devises under consideration. However, in order for the devise to cycle continuously, the condensate must be returned to the evaporator.
Hitherto in the considered known devises, the motive power for the liquid phase circulation has been derived mainly from one of two natural forces: the force of gravity, in a devise known as a thermosyphon; or, the force of capillary action, in a devise known as a heat pipe. When properly installed, either of these devises works well; however each has specific disadvantages.
Thermosyphons must be configured so that the condenser is located above the evaporator with respect to the gravitational field. In a system so configured, the condensate will run downhill to the evaporator. Conversely, if the evaporator is located above the condenser the working fluid liquid cannot circulate. This constraint precludes use of this system to transfer thermal energy in opposition to gravitational forces. Such devises cannot be used in instances where the heat-source is located above the heat-sink. Further, such devises will not work in an environment that lacks a gravitational field.
Heat pipes can transfer thermal energy in opposition to gravitational forces and they work well in zero-gravity or microgravity environments; however, such devices are constrained by the relative weakness of capillary force. In practice, the distance which capillary force can move liquid working fluid has been limited. The velocity of the liquid movement through a capillary passageway is necessarily slow, consequently the rate of thermal transfer is restricted. Further, a thermal overload will cause a vapor plug in the capillary structure which will shut down the thermal transfer until such time as the vapor can dissipate. As demonstrated in practice, drying of the wick limits the thermal transport capacity of a heat pipe system. This drying occurs when the liquid is boiled off at the evaporative region faster than the capillaries can return it from the condensing region.
Thermosyphons and heat pipes have advantages as well. In a properly installed system both gravitational and capillary forces are self-starting, in that the devise operation begins automatically when the temperature of the heat-source rises above the temperature of the heat-sink the necessary increment to begin the vapor phase thermal transfer. Such devises are self-regulating, so that within the limits of their transport capacity, the devise performance will automatically tend to match the thermal load. An additional advantage is that such devises do not require electrical energy to operate and thus may be used even though isolated from an electrical power source.
In known systems the working fluid circulates in an uninterrupted cycle. Such systems are characterized by the use of a single condenser chamber and, except for the circulation of the working fluid, do not have moving parts.
Under these circumstances, there is an increasing demand for a mechanism that will operate without active system controls and that retains advantages of the prior art, such as self-starting, self-regulating features. In addition, there is a demand for a devise that will transfer thermal energy in opposition to gravitational forces, that does not require the use of electricity, and that will transport thermal energy with great reliability.